Laser processing of solar cells with anti-reflective coating

ABSTRACT

Contact holes of solar cells are formed by laser ablation to accommodate various solar cell designs. Use of a laser to form the contact holes is facilitated by replacing films formed on the diffusion regions with a film that has substantially uniform thickness. Contact holes may be formed to deep diffusion regions to increase the laser ablation process margins. The laser configuration may be tailored to form contact holes through dielectric films of varying thicknesses.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/028,059, filed on Feb. 15, 2011, which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made with Governmental support undercontract number DE-FC36-07GO17043 awarded by the United StatesDepartment of Energy. The Government may have certain rights in theinvention.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto solar cell fabrication processes and structures.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. A solar cell includes P-type andN-type diffusion regions. Solar radiation impinging on the solar cellcreates electrons and holes that migrate to the diffusion regions,thereby creating voltage differentials between the diffusion regions. Ina back contact, back junction (BCBJ) solar cell, the P-type and N-typediffusion regions and the metal contacts coupled to them are on thebackside of the solar cell. The metal contacts allow an externalelectrical circuit to be coupled to and be powered by the solar cell.

In high-efficiency solar cells, cell parameters, such as shuntresistance, series resistance, and bulk lifetime are importantparameters to maintain on the final fabricated devices. Solar cellprocess steps, in particular laser ablation steps on BCBJ solar cells,may impact each of these parameters. Post laser losses due to seriesresistance or lifetime maybe be offset at the expense of step cost, suchas by adding thermal or etching steps. As is described within, an addedcomplication of shunting on high-efficiency BCBJ solar cells may beprevalent when the cell architecture has metal of one polarity overdiffusions of another polarity.

To compete with other energy sources available on the market, solarcells not only have to be efficient but also fabricated at relativelylow cost and high yield. Embodiments of the present invention pertain tonovel solar cell fabrication processes and structures that reduce thecost of solar cell fabrication and improve solar cell reliability.

BRIEF SUMMARY

In one embodiment, contact holes of solar cells are formed by laserablation to accommodate various solar cell designs. Use of a laser toform the contact holes is facilitated by replacing films formed on thediffusion regions with a film that has substantially uniform thickness.The film thickness as absorption may be tailored to match laserparameters. Dopant depth underneath contact holes may be controlled toincrease the laser ablation process margins. The laser configuration maybe tailored to form contact holes through dielectric films of varyingthicknesses.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter disclosed herein maybe derived by referring to the detailed description and claims whenconsidered in conjunction with the following figures, wherein likereference numbers refer to similar elements throughout the figures. Thefigures are not drawn to scale.

FIG. 1 schematically shows an example BCBJ solar cell with metalcontacts that are formed over opposite polarity diffusion regions.

FIG. 2 shows a top view of the solar cell of FIG. 1.

FIG. 3 shows a cross-section of the solar cell of FIG. 1 taken atsection A-A of FIG. 2.

FIGS. 4-6 show cross-sections of a solar cell being fabricated inaccordance with an embodiment of the present invention.

FIG. 7 shows another top view of the solar cell of FIG. 1.

FIG. 8 shows a cross-section of the solar cell of FIG. 1 taken atsection B-B of FIG. 7.

FIG. 9 shows a cross-section of a solar cell with deep diffusion regionsin accordance with an embodiment of the present invention.

FIGS. 10-13 show cross-sections of a solar cell being fabricated inaccordance with another embodiment of the present invention.

FIG. 14 shows a cross-section of a solar cell with laser-formed contactholes in accordance with another embodiment of the present invention.

FIG. 15 shows the cross-section of FIG. 3 with an additional dielectriclayer in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, processes, and structures, to provide athorough understanding of embodiments of the invention. Persons ofordinary skill in the art will recognize, however, that the inventioncan be practiced without one or more of the specific details. In otherinstances, well-known details are not shown or described to avoidobscuring aspects of the invention.

In some high-efficiency solar cell designs, metal contacts for onepolarity of diffusion region may run over an opposite polarity diffusionregion (e.g., metal contact for an N-type diffusion region formed over aP-type diffusion region). In that solar cell design, it is critical thatthe interlayer dielectric that electrically insulates the metal contactsfrom the diffusion regions is free of defects. Otherwise, a metalcontact of one polarity may electrically short to a diffusion region ofopposite polarity through a defect in the interlayer dielectric.

FIG. 1 schematically shows an example backside contact, backsidejunction (BCBJ) solar cell 300 with metal contacts that are formed overopposite polarity diffusion regions. In the example of FIG. 1, theP-type (labeled 352) and N-type (labeled 351) diffusion regions areformed in a substrate 401 (e.g., mono-crystalline or multi-crystallinesilicon). In other embodiments, the P-type and N-type diffusion regionsare formed in another layer, e.g., polysilicon, on a backside surface ofthe substrate of 401. Interlayer dielectrics are not shown in FIG. 1 forclarity of illustration.

The solar cell 300 includes metal contacts 301 and 303. Metal contacts301 are N-polarity metal contacts in that they electrically couple tocorresponding N-type diffusion regions. Similarly, metal contacts 303(only one is shown in FIG. 1) are P-polarity metal contacts thatelectrically couple to corresponding P-type diffusion regions. The metalcontacts 301 and 303 may be interdigitated. One metal contact 301 isdepicted in FIG. 1 as a transparent line tracing to more clearly showunderlying N-type diffusion regions. As shown in FIG. 1, an N-polaritymetal contact 301 passes over portions of a P-type diffusion region.This creates the possibility of the N-polarity metal contact 301 beingelectrically shorted to the P-type diffusion region through anintervening interlayer dielectric (not shown in FIG. 1; see 305 in FIGS.3 and 8).

FIG. 2 shows a top view of a portion of the solar cell 300. The solarcell 300 includes contact holes 302 that are formed through aninterlayer dielectric separating the N-polarity metal contact 301 fromunderlying diffusion regions. The N-polarity metal contact 301 contactsunderlying N-type diffusion regions through corresponding contact holes302.

FIG. 3 shows a cross-section of the solar cell 300 taken at section A-Aof FIG. 2. As shown in FIG. 3, the solar cell 300 includes an interlayerdielectric 305, which electrically insulates the N-polarity metalcontact 301 from underlying diffusion regions. Contact holes 302 areformed through the interlayer dielectric 305 to allow the N-polaritymetal contact 301 to electrically connect to corresponding N-typediffusion regions. The contact holes 302 are typically formed byconventional masking and wet etching. The inventors discovered that someetchants used in the etch process may worsen existing imperfections(e.g., pinholes, pits, and other defects) in the interlayer dielectric305, turning the imperfections into full-blown defects. For example,some etchants may enlarge existing pinholes. As another example, someetchants may result in creation of an electrical short 306 through theinterlayer dialect 305.

Using a laser, rather than a conventional wet etch process, to form thecontact holes 302 advantageously avoids worsening imperfections that maybe present in the interlayer dielectric 305. By avoiding exposure of theinterlayer dielectric 305 to harmful etchants during contact holeformation, a laser ablation step preserves the integrity of theinterlayer dielectric 305.

FIG. 4 shows a cross-section of a solar cell 300 being fabricated inaccordance with an embodiment of the present invention. The solar cell300 has a front side 153 and a backside 152. The front side 153 facesthe sun to collect solar radiation during normal operation. The backside152 is opposite the front side 153.

In the example of FIG. 4, the substrate 101 comprises an N-typemonocrystalline silicon wafer. The P-type and N-type diffusion regionsare formed in the solar cell substrate 101, but may also be in anotherlayer (e.g., polysilicon) formed on the solar cell substrate 101. Thefront side surface of the substrate 101 is textured with random pyramidsto increase solar radiation collection efficiency. A passivation region107 passivates the front side surface of the substrate 101 to minimizerecombination. In one embodiment, the passivation region 107 is anN-type passivation region formed by diffusing N-type dopants from thefront side 153. The N-type dopants may comprise phosphorus. In oneembodiment, the passivation region 107 is formed by heating thesubstrate 101 in a furnace where phosphorus is introduced. Thephosphorus diffuses into the front side of the substrate 101 to form thepassivation region 107. A silicon dioxide layer 108 on the back side 152of the solar cell is a byproduct of forming the passivation region 107.More specifically, the heating step to diffuse N-type dopants into thesubstrate 101 and form the passivation region 107 also results in growthof the oxide layer 108 on the backside surface of the substrate 101.

An anti-reflective coating 109 is formed on the front side 153 and ananti-reflective coating 110 is formed on the backside 152. In oneembodiment, the anti-reflective coatings 109 and 110 comprise siliconnitride. On the front side 153, the anti-reflective coating 109 isformed on the passivation region 107 on the front side surface of thesubstrate 101. On the backside 152, the anti-reflective coating 110 isformed on the oxide layer 108.

In FIG. 5, a laser ablation step is performed on the solar cell 300 toform contact holes to the P-type and N-type diffusion regions. The laserablation step may involve firing one or more laser beams to removematerials from the backside 152 and thereby expose the P-type and N-typediffusion regions for metallization. In the example of FIG. 5, the laserablation step removes portions of the anti-reflective coating 110 andoxide layer 108 to form contact holes to the P-type and N-type diffusionregions. The laser ablation step may be performed by firing laser beamsthrough a laser scanner, which scans the laser beams on the backside 152to form the contact holes. A commercially available laser source andscanner may be employed to perform the laser ablation. An example solarcell ablation system that employs a laser is disclosed in commonly-ownedU.S. application Ser. No. 12/829,275, filed on Jul. 1, 2010. Otherablation systems that employ a laser may also be employed.

The use of a laser to form the contact holes to the P-type and N-typediffusion regions advantageously eliminates masking and curing stepsthat may be necessary in other processes where the contact holes areformed by a traditional etch process. In addition, laser ablationprevents exposure of the anti-reflective coating 110 and oxide layer108, and any interlayer dielectric that may be present, to etchants thatmay worsen existing defects or imperfections.

In FIG. 6, metal contacts 112 and 113 are formed in the contact holes tomake electrical connection to corresponding diffusion regions. In theexample of FIG. 6, the metal contacts 112 are formed in contact holes tomake electrical connection to the P-type diffusion regions. Similarly,the metal contacts 113 are formed in contact holes to make electricalconnection to the N-type diffusion regions. The metal contacts 112 and113 may be interdigitated, and may comprise copper or other single layeror multi-layer electrically conductive materials employed formetallization. The metal contacts 112 and 113 may be formed byelectro-plating, for example. The metal contacts 112 and 113 allow anelectrical circuit to be coupled to and be powered by the solar cell. Ametal contact 112 to a P-type diffusion region may pass over an N-typediffusion region. Similarly, a metal contact 113 to an N-type diffusionregion may pass over a P-type diffusion region. Because the metalcontacts are formed in contact holes formed by laser ablation, thechances of a metal contact electrically shorting to an opposite polaritydiffusion region is greatly diminished.

A potential laser-related problem discovered by the inventors is nowdescribed with reference to FIGS. 7 and 8. FIG. 7 shows another top viewof a portion of the solar cell 300 of FIG. 1. The solar cell 300includes contact holes 307 that are formed through an interlayerdielectric separating the P-polarity metal contact 303 from underlyingdiffusion regions.

FIG. 8 shows a cross-section of the solar cell 300 taken at section B-Bof FIG. 7. Contact holes 307 (i.e., 307-1, 307-2, . . . ) are formedthrough the interlayer dielectric 305 to allow the P-polarity metalcontact 303 to electrically connect to the underlying P-type diffusionregion.

In the example of FIG. 8, the contact holes 307 are formed by laserablation. If the laser is not properly controlled, the laser beam maypunch through the diffusion region, thereby adversely affecting theoperation of the solar cell by electrically shorting the subsequentlyformed metal contact to the substrate. In the example of FIG. 8, thelaser ablation step formed the contact hole 307-1 all the way throughthe interlayer dielectric 305, all the way through the P-type diffusionregion, and into the substrate 401. One way of addressing this laserpunch through problem is to make the diffusion regions deeper, as nowexplained with reference to FIG. 9.

FIG. 9 shows a cross-section of a solar cell 400 with deep diffusions inaccordance with an embodiment of the present invention. In the exampleof FIG. 9, a P-type diffusion region (labeled as 402) is formed in asolar cell substrate 411, which comprises a monocrystalline siliconwafer. In other embodiments, the P-type diffusion region is formed inanother layer (e.g., polysilicon) formed on the backside surface of thesubstrate 411. In the example of FIG. 9, contact holes 405 (i.e., 405-1,405-2, . . . ) are formed through an interlayer dielectric 403 by laserablation. A P-polarity metal contact 404 electrically connects to theP-type diffusion region through the contact holes 405. It is to be notedthat all figures in this disclosure, including FIG. 9, are not drawn toscale.

In the example of FIG. 9, the P-type diffusion region is formed to berelatively deep. For example, the P-type diffusion region may have adepth 407 deeper than 0.5 μm. The depth of the P-type diffusion regionis dictated by the process margins of the laser ablation step.Preferably, the required laser ablation depth is minimized for theprocess, and then measured on a cross-section. The dopant depth of thediffusion region is then set deeper than the required laser ablationdepth by controlling the dopant formation process (e.g., furnacetemperature and time, starting dopant concentration, etc). Deepdiffusion regions advantageously allow for a laser ablation step withwider process margins. Deep N-type diffusion regions formed on thebackside of the solar cell with the P-type diffusions region may alsohave the same depth as the P-type diffusion regions.

In the example of FIG. 9, the contact hole 405-1 is formed relativelydeep into the P-type diffusion region. The deep contact hole 405-1 maybe due to problems related to process control in general, laser ablationprocess margin, or other issues. However, unlike in FIG. 8, the contacthole 405-1 does not punch all the way through the P-type diffusionregion because of the depth of the P-type diffusion region. The metalcontact 404 is formed in the contact holes 405 (i.e., 405-1, 405-2, . .. ). The metal contact 404 may safely pass over a diffusion region ofopposite polarity (i.e., N-type diffusion region) because the metalcontact 404 is formed in contact holes formed by laser ablation.

The inventors also discovered that different film thicknesses found insome solar cell designs may complicate laser ablation. An example ofsuch solar cell design is shown in FIG. 10.

FIG. 10 shows a cross-section of a solar cell 420 having a non-uniformfilm 423 through which contact holes are to be formed. In the example ofFIG. 10, the film 423 comprises an interlayer dielectric. The film 423may be a single layer dielectric or a multi-layer dielectric stack(e.g., oxides and/or nitrides; oxides and/or polyimide) formed over asolar cell substrate 421. The solar cell substrate 421 may comprise amonocrystalline silicon wafer. The P-type and N-type diffusion regionsmay be formed in the solar cell substrate 421 or in another layer (e.g.,polysilicon) formed on the solar cell substrate 421.

In the example of FIG. 10, portions of the film 423 over the P-typediffusion regions are thicker than portions of the film 423 over theN-type diffusion regions. In other cases, portions of the film 423 overthe N-type diffusion regions are thicker than portions of the film 423over the P-type diffusion regions. This difference in film thicknessesmay be due to the process of forming the P-type and N-type diffusionregions, such as in the sequence of forming dopant sources over thediffusion regions. Forming contact holes through the film 423 to theN-type diffusion regions requires less laser energy compared to formingcontact holes through the film 423 to the P-type diffusion regions.Using the same laser energy to form contact holes to the P-type andN-type diffusion regions may thus result in punching through the P-typediffusion regions, or other problems. On the other hand, using differentlaser energies to form contact holes to the P-type and N-type diffusionregions may require multiple laser ablation steps and may result inprocessing delays not just because of the additional steps, but also inreconfiguring the laser for different energies.

For the solar cell design of FIG. 10, the thickness of the dielectricstack over the P-type diffusion regions may be in the 500-10000Angstroms range, and the diffusion depth of the P-type diffusion regionsmay be in the 200-2000 nm range. For a high-efficiency solar cell, i.e.,a solar cell with efficiency greater than 20%, the standard bulkrecombination rate (BRR) and saturation current density (Jo) would beless than 1000 Hz and 120 fA/cm² if there were no laser damage. To avoidablation all the way through the junction in the base and increase theBRR and Jo, while also completely removing the film being ablated, theproper laser condition must be used. Using a wavelength shorter than 540nm while keeping the absorption depth to a minimum prevents the BRR fromincreasing higher than 1000 Hz. Using a laser with a pulse lengthshorter than 20 ps will keep the thermal ablation depth to less than2000 nm. The laser energy would then be tuned so that the ablationthreshold is achieved (e.g., 1-20 μJ). Complete oxide removal would thenresult in series resistance of less than 1 ohm-cm² in the finished solarcell. However, with these film stack thickness conditions on ahigh-efficiency solar cell, a single laser pulse will still not be ableto clear an entire dielectric stack without increasing the BRR and Jo.That is, keeping the BRR to less than 1000 Hz and Jo to less than 120fA/cm² will result in series resistance greater than 1 ohm-cm², andgetting the series resistance less than 1 ohm-cm² will result in the BRRincreasing higher than 1000 Hz. This problem may be solved by using 2 ormore laser pulses, where the pulse to pulse spacing is separated by lessthan 500 ns and the amplitude of the subsequent pulses is between 10%and 100% the amplitude of the first pulse. This allows for more materialremoval without additional increase in BRR and Jo. An examplemulti-pulse laser ablation process is described in commonly-owned U.S.application Ser. No. 12/795,526, filed on Jun. 7, 2010, and incorporatedherein by reference in its entirety. Other multi-pulse laser ablationprocesses may also be used.

Because the dielectric stack thicknesses over the P-type and N-typediffusion regions may be different, and thus require different laserenergies to achieve the proper BRR/series resistance balance, the laserablation tool gets relatively complicated, requiring changes in powerfor different regions of the solar cell being fabricated. This requiresprecise spatial coordination between the laser and the beam deliverysystem to synchronize laser power and location and avoid creating shunts(i.e., electrical shorts) due to a misaligned laser. Misalignment can beavoided by slowing down the beam delivery system. However, doing sowould result in lower throughput on the tool, and therefore increase thetool cost for a certain throughput. As a solution, the dielectric stackmay be tuned so that the ideal laser parameter, such as energy andnumber of pulses, on one region does not result in ablation in anotherregion. For example, dielectric stack thickness over the P-typediffusion regions may be made to be 5000-10000 Angstroms, and thedielectric stack thickness over the N-type diffusion regions may be madeto be less than 2500 Angstroms. This allows a laser energy of 3 μJ withtwo pulses to ablate the dielectric stack over the N-type diffusionregions, but not the dielectric stack over the P-type diffusion regions.

In any case where laser misalignment may cause a shunt problem asdescribed above (e.g., in FIG. 3, the electrical short 306), theinventors have discovered that an additional dielectric layer may bedeposited in a patterned way so that the laser is blocked from causingablation. FIG. 15 shows the cross-section of FIG. 3 except for theaddition of an additional dielectric layer 355 patterned on portions ofthe interlayer dielectric layer 305 over the P-type diffusion regions.Other components shown in FIG. 15 have been discussed with reference toFIG. 3.

In the example of FIG. 15, the additional dielectric layer 355 maycomprise a material that may be ablated sacrificially, such as apigmented ink. The additional dielectric layer 355 may be thick enough(e.g., greater than 500 Angstroms) to prevent absorption of the laserwavelength used. The additional dielectric layer 355 may also comprise amaterial that is transparent to the laser (e.g., polyimide) but thickenough (e.g., greater than 500 Angstroms) to prevent the ablatedmaterial underneath from breaking through. The additional dielectriclayer 355 may also comprise a semi-transparent material, provided thatthe combination of direct ablation of the sacrificial layer and ejectedmaterial from below does not cause a pinhole to form in the additionaldielectric layer 355. It should be noted that this additional dielectriclayer 355 may also have properties that prevent dielectric breakdown, asdiscussed later below.

In accordance with an embodiment of the present invention, the solarcell 420 of FIG. 10 is prepared for laser ablation by removing the film423 and any other material previously formed on the P-type and N-typediffusion regions. This approach is especially advantageous in caseswhere the dielectric stacks vary from each other by more than 200Angstroms. This approach is further illustrated in FIG. 11 where allmaterials on the P-type and N-type diffusion regions have been removedto expose the backside surface of the P-type and N-type diffusionregions. For example, the film 423 of FIG. 10 may be removed using aconventional wet etch process. The film 423 and any other material onthe P-type and N-type diffusion regions are removed to control thethickness of the film subsequently formed on the P-type and N-typediffusion regions. Accordingly, in the example of FIG. 12, asubstantially uniform film 424 is formed on the P-type and N-typediffusion regions. In essence, the film 424 replaces the non-uniformfilm 423. The film 424 may comprise an interlayer dielectric (e.g.,deposited or thermally grown oxide, followed by silicon nitride) that isdeposited with substantially uniform thickness. The film 424 may bedeposited by chemical vapor deposition, other deposition, or growthprocess that allows for uniform film deposition. In FIG. 13, thereplacement of the non-uniform film 423 with the uniform film 424 issubsequently followed by a laser ablation step to form contact holesthrough the film 424 to expose portions of the P-type and N-typediffusion regions. The contact holes allow metal contacts toelectrically connect to corresponding diffusion regions. A metal contactto a P-type diffusion region may pass over an N-type diffusion region.Similarly, a metal contact to an N-type diffusion region may pass over aP-type diffusion region. Because the metal contacts are formed incontact holes formed by laser ablation, the chances of a metal contactelectrically shorting to an opposite polarity diffusion region isgreatly diminished.

Contact holes through the film 423 of FIG. 10 may also be formed byappropriate control of the laser used in the laser ablation step.Typical ablation of dielectric films is through the process of indirectablation, where the laser energy is absorbed in the substrate, and thefilm is ejected via the outward force of the ablated substrate. Thistype of film ablation is known as indirect ablation. For example, whenthe film of interest does not interact strongly with the laserwavelength, ablation depth and damage in the substrate are drivenprimarily by pulse length, wavelength, and number of pulses of thelaser, all of which need to be reduced for minimal substrate ablationdepth. If the film or one of the films in a film stack of interestinteracts strongly with the laser wavelength, the laser processparameters will need to be adjusted accordingly, for example, byincreasing the number of pulses or by switching the laser wavelength sothat direct ablation occurs. Certain types of films may be removed viadirect ablation, without ablation in the silicon, by using multiplepulses. An example laser ablation process using multiple laser pulses isdescribed in commonly-owned U.S. application Ser. No. 12/795,526, filedon Jun. 7, 2010, and incorporated herein by reference in its entirety.Other multi-pulse laser ablation processes may also be used withoutdetracting from the merits of the present invention.

A method to modify the optical properties of a dielectric layer (e.g.,P-type or N-type doped silicon dioxide) or dielectric stack to suitlaser ablation parameters may include tuning refractive index andabsorption coefficients of the dielectric through compositional control,or by adding absorbing compounds to the dielectric layer to tune thedielectric layer to get either direct or indirect ablation. As aparticular example, refractive indices less than 2.0 for laserwavelengths of 530 nm or longer cause indirect ablation to occur andprevent residual material from remaining on the substrate.

As applied to FIG. 10, a first laser ablation step may be performed toform contact holes through portions of the film 423 over the P-typediffusion regions. The first laser ablation step may be in accordancewith a first laser configuration having parameters tailored specificallyfor the characteristics of the portions of the film 423 over the P-typediffusion regions. A second laser ablation step may be performed to formcontact holes through portions of the film 423 over the N-type diffusionregions. The second laser ablation step may be in accordance with asecond laser configuration having parameters tailored specifically forthe characteristics of the portions of the film 423 over the N-typediffusion regions. The first configuration being different from thesecond configuration. For example, the first configuration may involvethe laser firing multiple laser pulses to drill through portions of thefilm 423 over the P-type diffusion regions. As another example, thesecond configuration may involve the laser firing a single laser pulseto drill through portions of the film 423 over the N-type diffusionregions.

The resulting structure is schematically shown in FIG. 14, where thecontact holes 435-1 and 435-2 through the film 423 and exposing theP-type diffusion regions are formed by laser ablation with the laserfiring in accordance with the first configuration, and the contact hole435-3 through the film 423 and exposing an N-type diffusion region isformed by laser ablation with the laser firing in accordance with thesecond configuration. Metal contacts may be formed in the contact holes435 (i.e., 435-1, 435-2, 435-3). A metal contact may be safely formedover a diffusion region of opposite polarity (e.g., N-polarity metalcontact over a P-type diffusion region) because the metal contacts arein contact holes formed by laser ablation.

In another embodiment, where defects in an interlayer dielectric, suchas the one described with reference to FIG. 3, may be present, theanti-reflective coating deposited on the backside (e.g., anti-reflectivecoating 110 of FIGS. 4-6) may be tailored in a way to improve thedielectric integrity of the back stack. For example, the thicknessand/or resistivity of the backside anti-reflective coating may beincreased by approximately 50-100 Angstroms. As another example, theanti-reflective coating may comprise two layers, such as a layer ofamorphous-silicon that is uniformly deposited on top or underneath asilicon nitride layer. Preferably, to save fabrication cost, the layerof amorphous silicon and the silicon nitride layer are formed in-situ(i.e., same loading) in the same process step in the same tool. The useof a two layer anti-reflective coating as described hereinadvantageously increases not just the thickness of the anti-reflectivecoating but also its dielectric constant, thereby facilitating laserablation.

In reverse bias, for example, upwards of 6 volts may be applied acrossthe interlayer dielectric film. Typical plasma-enhanced chemical vapordeposition (PECVD) nitride films having a thickness in the range ofabout 400 Angstroms would breakdown at this voltage if the voltage wereapplied locally. A target breakdown field of the dielectric film forsuch an application can be greater than 1×10⁷ V/cm. The target breakdownfield may be achieved by addition of 50-100 Angstrom layer of amorphoussilicon to the silicon nitride layer, which could decrease the effectivefield applied within the stack.

Improved processes and structures for fabricating solar cells have beendisclosed. While specific embodiments of the present invention have beenprovided, it is to be understood that these embodiments are forillustration purposes and not limiting. Many additional embodiments willbe apparent to persons of ordinary skill in the art reading thisdisclosure.

What is claimed is:
 1. A process of fabricating a solar cell, the process comprising: forming an anti-reflective coating over an interlayer dielectric on a backside of a solar cell, the anti-reflective coating together with the interlayer dielectric being configured to have a breakdown voltage that is greater than 1×10⁷ V/cm; using a laser to form a contact hole through the interlayer dielectric and the anti-reflective coating to expose an underlying diffusion region of the solar cell; and forming a metal contact in the contact hole to electrically connect to the diffusion region, wherein the anti-reflective coating comprises a layer of amorphous silicon and a layer of silicon nitride.
 2. The process of claim 1 wherein the breakdown voltage is configured by decreasing a refractive index of the interlayer dielectric to less than 1.95.
 3. A solar cell fabricated using the process of claim
 1. 4. A process of fabricating a solar cell, the process comprising: forming a multi-layer anti-reflective coating (ARC) over an interlayer dielectric on a backside of a solar cell, the multi-layer ARC comprising a first ARC layer and a second ARC layer, the multi-layer ARC and the interlayer dielectric layer being configured to have a breakdown voltage that is greater than 1×10⁷ V/cm; using a laser to form a contact hole through the interlayer dielectric, the first ARC layer, and the second ARC layer to expose an underlying diffusion region of the solar cell; and forming a metal contact in the contact hole to electrically connect to the diffusion region, wherein the first ARC layer comprises silicon nitride that is formed over the second ARC layer that comprises amorphous silicon.
 5. The process of claim 4, wherein the first ARC layer and the second ARC layer are formed in-situ.
 6. The process of claim 4, wherein the multi-layer ARC is formed over a plurality of N-type diffusion regions and a plurality of P-type diffusion regions on the backside of the solar cell.
 7. The process of claim 4, wherein the breakdown voltage is configured by decreasing a refractive index of the interlayer dielectric to less than 1.95. 